Electroless deposition of metals is a process in which the deposition of the metal takes place without the use of external current. The term electroless plating is not very precise. Both autocatalytic reduction and immersion deposition often are referred to as electroless plating. Electroless gold plating has an advantage over electroplated gold due to its ability to plate parts which have discreet and isolated areas; whereas, the electroplating techniques are difficult or impossible to utilize under such conditions.
Immersion or displacement occurs when one metal displaces another from the solution. This displacement is controlled by the potential or reduction potential of the metals under the reaction conditions. Generally, metals with negative potentials (active metals) have a greater tendency to form ions in solutions than those with less negative potentials, i.e., more positive. This process ceases after the surface of the bare metal is completely coated; however, in some cases the thickness of the deposits is thicker than expected for molecular deposit. This behavior can be explained on the basis that the mechanism of the displacement reaction at the surface of the metal is not homogeneous in nature. If the surface consists of areas which are more active and then others which are less active (more noble), the more active sites form anodic centers while the less active ones form a cathodic center. Therefore, immersion deposition is a galvanic displacement reaction with a mixed potential reaction consisting of cathodic and anodic half-reactions in much the same manner as a corrosion reaction. At any instant, during the reaction the cathodic and anodic sites must be distributed side by side on a microscopic scale on the substrate surface. Accordingly, gold will deposit at the cathodic sites while the substrate oxidation will take place at the anodic site. It is generally recognized that for any metal deposition system, a strong atom-to-surface interaction will result in the formation of a high density of nuclei, while a weak interaction will give widely spaced nuclei. The deposits obtained from the galvanic displacement are usually a porous deposit.
In recent years a fairly substantial literature has developed with respect to the electroless method of gold plating on surfaces. U.S. patents of special interest both as to the electroless gold plating method and the problems associated with this procedure include; U.S. Pat. Nos. 3,589,916 (McCormack); 3,697,296 (Bellis); 3,700,469 (Okinaka); 3,917,885 (Baker); as well as the earlier patents and articles cited therein. Relevant articles include: Rich, D. W., Proc. American Electroplating Society, 58 (1971); Y. Okinaka, Plating 57, 914 (1970); and Y. Okinaka and C. Wolowodink, Plating 58, 1080 (1971). This body of literature is pertinent to the present invention insofar as it discloses alkali metal cyanides as the source of the gold or related metal in the electroless bath as well as the use of alkali metal borohydrides and amine boranes as reducing agents. Thus, for example, the 1970 article by Okinaka as well as his U.S. Pat. No. 3,700,469 describes an electroless gold plating bath having the following ingredients:
(1) soluble alkali metal gold complex;
(2) excess free cyanide such as potassium cyanide;
(3) an alkaline agent such as potassium hydroxide; and
(4) a borohydride or an amine borane.
The 1971 article by Okinaka et al. as well as Baker's U.S. Pat. No. 3,917,885 point out the problems associated with the use of these particular plating baths, particularly when the cyanide concentrations increase. Other problems were encountered when bath replenishment was carried out and the instability of the baths when the plating rate of about 2.5 microns was approached. There was also a need to avoid undesirable gold precipitation from the baths.
In U.S. Pat. No. 3,917,885 the problems noted above were overcome by utilizing, as the gold or related metals source, an alkali metal imide complex formed from certain special imides. In order to maintain the electroless gold plating at the desired pH of about 11 to 14, the Baker patent suggests the addition to the bath of alkali metal buffering salts such as the citrates, etc.
It is also possible to classify the prior art pertaining to immersion gold deposition into two categories, based on the pH of the bath: